qverton



Jan. 31, 1961 w. c. OVERTON, JR Re. 24,931

ACOUSTIC wsu. LOGGING WITH END SHIELDING Original Filed March 12, 1952 4Sheets-Sheet 1 w a w m N R. n O a Q m m M f m R E m .w w 7 r R I O I A.R m E C w W M 5 6 W .G M M M M d R r, 1| 5 w w M RE 5:. m u. w 1 M wuL5 mp M 4 A A 1 W E m y m w Mm c v 3 M 2 A 7 A 4 P A 2" n M i w M w W 8Jan- 31, 19 1 w. c. OVERTON, JR 24,931

ACOUSTIC WELL LOGGING WITH END SHIELDING 4 Sheets-Sheet 2 WILLIAM C.OVERTON JR.

. INVENTOR.

Original Filed March 12, 1952' ATTORNEY w. c. OVERTON, JR Re. 24,931

ACOUSTIC WELL LOGGING WITH END SHIELDING l 6 9 1 L 3 m a 4 Sheets-Sheet3 Original Filed March 12, 1952 lam] Fig. 4

WILLIAM C.OVER TON JR.

POWER AMPLIFIER INVENTOR.

ATTORNEY Jan. 31, 1961 w, Q QVERTQN, JR Re. 24,931

ACOUSTIC WELL LOGGING WITH END SHIELDING Original Filed March 12, 1952 4Sheets-Sheet 4 POWER AMPL/F/[R @560000? q} AC SOURCE INIEGQAT/NGAMPLIFIER WILLIAM C. OVER ro/v JR.

INVENTOR BY gwm ATTORNEY States ACOUSTIC WELL LOGGING WITH END SHIELDINGWilliam C. Overton, Jr., Prince Georges County, Md, assignor, by mesneassignments, to Socony Mobii Oil Company, Inc., a corporation of NewYork Original No. 2,878,886, dated Mar. 24, 1959, Ser. No. 276,095, Mar.12, 1952. Appiication for reissue Oct. 21, 1959, Ser. No. 847,855

16 Claims. (Cl. Isl-.5)

This invention relates to acoustic well logging and more particularly tothe use of cylindrical waves in a bore hole substantially free ofend-effects. In a more specific aspect the invention relates to thegeneration of cylindrical waves for transmission to formations adjacenta bore hole and the measurement of characteristics of a selected portionof the cylindrical wave thus generated.

It is known to log various earth formations penetrated by a bore hole bymoving through the bore hole an acoustic generator and measuring thereaction of the formations upon the generator. For example in the Patent2,530,971 to Kean there is disclosed a system in which symmetric wavesare generated in a bore hole, the waves having a wave length that islong compared with the length of the transmission path through liquidfilling an annular space between the acoustic generator and the walls ofa bore hole.

From a study of the propagation of acoustic energy from a long axiallysymmetrical cylindrical source radiating sound waves through a bore holeliquid and thence to surrounding earth formations penetrated by the borehole, in the general case of arbitrary [acoustice] acoustic wavefrequencies utilizing approximate boundary conditions at the Wall of thebore hole and at the cylindrical source radiating into the bore holeliquid, it has been found that the solution for acoustical energy andthe mechanical driving point impedances at the wall of the cylindricalsource are complicated indeed. The functions contributing to theforegoing factors are density, shear modulus and bulk modulus of theearth formations adjacent the source, and the density and bulk modulusof the bore hole liquid. The conditions encountered in actual fieldoperations make analysis most difi'icult.

However for a low frequency case, that is for frequencies less than 200cycles per second, the complicated solutions for the energy imparted tothe formations and for the mechanical driving point impedances aregreatly simplified. For the low frequency case a simple relation shipbetween density and shear modulus may be recorded directly.

However, in both the general case or the restricted low frequency casethe acoustic energy and the mechanical driving point impedance arereadily determinable only if the cylindrical source does not have finitedimensions. The character of the wave radiating from the center of thesource is truly cylindrical but at the ends the radiation isnon-cylindrical, approaching spherical Waves and requires anunderstanding of end radiation effects and a correction therefor if thefunctions recorded are to be completely understood. Although correctionscan be applied by computational methods, the corrections may vary in anindeterminate way if the logging instrument is moved past earthformations of varying density and shear Re. 24,931 Reissued Jan. 31,1961 modulus. Thus the desirability of providing an acoustic loggingsystem independent of end radiations even though of finite length ismost desirable.

By the present invention there is provided an improved acoustic loggingsystem in which axially symmetrical cylindrical waves are generated andcharacteristics of a predetermined portion of the wave are measured.More particularly, applicant provides a method for acoustic logging inwhich an exploring element is used to log a liquid filled bore hole. Themethod includes the steps of driving the exploring element to generatean axially symmetric wave with the element in a homogeneous medium inwhich condition the amplitude of the wave is adjusted to a predeterminedlevel over substantially the entire length of the element so that overan intermediate section of the element the axially symmetric Wave iscylindrical. The element is then moved through the bore hole past theformations adjacent thereto to modify the wave over the length of theelement in dependence upon the acoustic properties of the formations andvariations in the wave over a preselected intermediate fraction of thelength of the element are measured for determination of variations inthe acoustic properties of the formations.

In a more specific aspect of the invention a first portion of theexploring element is driven from a low frequency energy source togenerate an axially symmetric wave acoustically to couple formationsadjacent the latter portion to the exploring element. Portions of theexploring clement above and below the first portion are driven in phaseWith the first portion for generation of axially symmetric Waves tocouple adjacent formations to the portions of the element above andbelow the first portion. T he amplitude of the Waves from all portionsof the element are then adjusted to substantially the same level toassure a wave cylindrical in form adjacent the first portion. Theexploring element is then moved through the bore hole to modify thewaves from the several portions of the element in dependence upon theacoustic properties of the formations, and variations in the wave fromthe first portion are measured for determination of variations in theacoustic properties of the formations adjacent the first portion only.

in accordance with a further aspect of the invention, applicant providesa plurality of radiating members supported in an end-to-end relationwith means for driving the radiating members in phase to generateaxially sym metric acoustic waves in the formations adjacent thereto.The system is provided with means for adjusting the amplitude of thewaves from each of the radiating members to a preselected value. The endmembers serve as acoustic guards for intermediate members so that as theexploring system is moved through the bore hole the intermediate membersradiate energy substantially independent of end-effects, andmeasurements made of the characteristics of the wave produced by theintermediate members are then independent of end-effects and dependentprincipally upon a restricted section of the formations adjacent onlythe intermediate member.

Further applicant provides automatic power controlling means for each ofthe individual radiating members. Means responsive to the amplitude ofvibration of each of the members produce voltages which are utilized tocontrol the power delivered to the several members. The power requiredto maintain constant the amplitude of a cylindrical wave of lowfrequency is dependent principally upon formation density and shearmodulus. Applicant provides a means for measuring variations in thepower required to maintain constant the amplitude of a cylindrical wave,the wave being maintained cylindrical over a discrete measuring sectionby utilization of the end guard radiators.

For a further understanding of the present invention and for a moredetailed description thereof, reference may now be had to the followingdescription taken in conjunction with the accompanying drawings inwhich:

Fig. 1 [is a diagrammatic representation of] illustrates an end shieldedacoustic logging system;

Fig. 2 is an enlarged view illustrating details of a portion of theradiator of Fig. 1;

Fig. 3 is a sectional view taken along line 3-3 of Fig. 2;

Fig. 4 is a schematic diagram of a portion of the driving system; and

Fig. 5 is a modification of the invention.

Referring now to Fig. 1, there is illustrated one form of an exploringtool in the form of an elongated acoustic radiating element whichincludes a rubber sheath completely enclosing a plurality of cylinders1119. The cylinders 11-19 are supported in an end-to-end relation andtogether form a composite elongated cylindrical radiator. The rubbersheath, whose periphery 10 has been indicated by a dotted outline, hasbeen broken away in order more clearly to show the associated elementsof the system.

The cylindrical elements 11--19 preferably are piezoelectric crystals,for example of barium titanite, having a first electrode coated on theinner surface of each cylinder and a second electrode coated on theouter surface of the cylinder. Upon application of an electricalpotential to the electrodes, the radial dimensions of the crystalschange. When a signal is simultaneously applied to all of the elements1119, there is produced a cylindrical wave symmetrical to the axis ofthe bore hole 20, causing acoustic energy to travel through liquids inthe bore hole surrounding the sheath 10 and into the formations adjacentthe bore hole 20.

In accordance with the present invention, the radiator comprised of theelements 1119 is divided into a plurality of discrete elements each ofwhich is separately excited by energy preferably from a low frequencyalternating current source. As illustrated, an alternating currentsource 30 whose output preferably is sinusoidal is connected by way of afirst channel 31 to a power amplifier 32. The output of the amplifier 32is then applied by way of channel 33 to each of the cylinders 11, 12 and13. The cylinders are connected in parallel to the channel 33 so thatthe AC. voltage from the amplifier 32 produces sinusoidal variations inthe dimensions of cylinders 11-13.

Similarly, the source 30 is connected by way of channel 35 to poweramplifier 36. An output channel 37 leading from amplifier 36 isconnected to the centrally located elements 14, and 16, the latter beingparallel connected to channel 37. A third channel 40 is con nectedbetween the source 30 and a third power amplifier 41. The output channel42 from amplifier 41 is connected to the lower elements 17, 18 and 19.

It will thus be seen that the bore hole unit is comprised of a firstsection 11, 12, 13 driven from a first amplifier 32, a second orintermediate section 14, 15, 16 driven from a second amplifier 36, and athird or lower section 17, 18, 19 driven from a third amplifier 41 wherethe amplifiers 32, 36 and 41 are driven from the common source 30.Therefore the elements 11--19 are driven in phase so that the waveproduced in a homogeneous medium would be the same as produced by asingle radiating element of corresponding length, By individuallydriving discrete sections, it is possible to measure characteristics ofthe wave, or of a selected portion of the wave, without introduction ofindeterminate errors due to radiation from the ends of the system.

For example, it has been determined that the total energy required todrive a selected portion of a radiator such as illustrated in Fig. 1 hasthe following relation.

at low audio frequencies to parameters of the earth formations:

r. p2 4 F where:

The foregoing equation is the low frequency approximation of a generalsolution of a cylindrical radiating system valid for frequencies below200 cycles per second for cylindrical waves symmetrical with respect tothe axis of the bore hole 20 [10]. It will be seen that if the bore holeradius (b) remains constant and if the amplitude of the cylindricalwaves also is maintained constant by operation of the system, the onlyvariables effecting the total energy wil be the density p and the shearmodulus p2 of the adjacent formations. A log dependent upon suchparameters presents in bold relief the location of interfaces betweenformations of different character.

The wave produced by the system of Fig. 1 is not a cylindrical wave inits entirety for the reason that the r radiating system has finitelength. However applicant provides the end shielding members 11, 12, 13,17, 18 and 1? for the central section 14, 15 and 16 so that the energyrequired to drive the central elements 14, 15 and 16 at a constantamplitude while moving the exploring tool throughout the length of thebore hole will be the same as if the Wave were entirely cylindrical. Theenergy then varies directly as the density of the formations adjacentthe intermediate or central section and inversely as the shear modulusof the formations. In Fig. 1 a wattmeter Ethermocouple element] 45 isconnected in circuit with the channel 37 and to a recorder 46 in suchmanner that the output signal produced by the wattmeter isrepresentative of [voltage produced by the thermocouple element isproportional to] the power output of the amplifier 36.

In order to maintain constant the amplitude of vibration of the elements14, 15 and 16 the electrode on the outer surface of the cylinder 15 ismilled or grooved at a point intermediate the ends thereof and adetecting circuit including conductors 56 connected to the insulatedsection detects the voltage produced by variation of the dimension ofthe cylinder. The cylinder 15 then serves both as a driver and detectoras will be described in greater detail in connection with Fig. 2.Thevoltage on channel 50 is applied to an amplifier 51 whose outputcircuit is connected by Way of channel 52 to an automatic volume controlcircuit 53. The output of the automatic control circuit is connected byway of channel 54 in a gain controlling relation (later described indetail) to the power amplifier 36.

In a similar manner a voltage detected in the upper section, from thecylinder 12, is applied by way of channel 60 to an amplifier 61.Amplifier 61 is connected by way of channel 62 to a second AVC circuit63 for applying by way of channel 64 a gain control voltage to the poweramplifier 32. The voltage appearing on channel 70, proportional to theamplitude of vibration of the cylinder 18 is applied to amplifier 71.The output of amplifier 71 is connected by way of channel 72 to a thirdAVC circuit 73 for applying by way of channel 74 a gain control voltageto the power amplifier 41.

In operation, with the exploring tool including the cylinders 11-19 in ahomogeneous medium or in air, the low frequency sinusoidal signal isapplied from source 30 by way of amplifiers 32, 36 and 41 to thecylinders 11--19 to produce axially symmetric acoustic waves in theadjacent formations. The AVC circuits 53, 63 and 73 are adjusted forequal amplitudes of vibration of the three sections of the radiatingunit. The unit is then lowered through the bore hole to acousticallycouple adjacent formations to the system. Since the levels of theoutputs of amplifiers 32, 36 and 41 were initially adjusted for equalamplitudes of vibration of the associated crystals, the portion of thewave produced by the central section 14, 15, 16 will be a truecylindrical Wave and the power required to maintain constant theamplitude of the wave produced by that portion of the radiating systemvaries principally in dependence upon density and shear modulus.

As the system is lowered into the bore hole past different formations,the AVG circuits 53, 63 and 73 automatically maintain the amplitude ofvibration of all elements constant so that 'the character of the wave istruly cylindrical over the length of the bore hole corresponding to thecentral section of the radiator. Reliance may then be placed upon themeasurement of the energy to the central section 14, 15, 16.

In Fig. l, the electrical components, the source, amplifiers and the AVGcircuits have been illustrated in a manner most convenient to anunderstanding of the operation of the system. It is to be understoodthat the components may be positioned at the surface of the earth withthe recorder 46 and connected to the bore hole system by way of amulti-conductor cable. Alternatively selected portions of the electricalsystem may be housed in a suitable pressure resistant casingmechanically secured to the upper end of the rubber sheath 10. Suchexpedients are well known and understood by those skilled in the art andhave therefore not been described in detail.

Referring now to Fig. 2, there is illustrated a portion of the radiatingsystem of Fig. 1 showing the cylindrical crystal 15 with the mechanicalsupporting means and electrical connections therefor. Crystal 15 issupported by a plurality of rods 50, 85 and 86. Rod 81? passes through alug 81 best illustrated in Fig. 3. Lug 81 is an integral part of asupporting washer 82 which also has lugs 83 and 84 positioned 120 alongthe periphery of washer 82 from lug 81 and from each other. Theadditional supporting rods 85 and 86 pass through openings in the lugs83 and 84. Referring again to Fig. 2, spacers 3B and 89 are positionedon opposite sides of the washer 82 to cushion the associated crystals.Spacers 8S and 89 preferably are of rubber. Crystal 15 is spaced fromthe washer 82 by spacer 89. Similarly the cylinder 16 is spaced from thewasher 82 by the spacer 88. Rods 80, 85 and 86, Fig. 2, extending thefull length of the radiating system of Fig. 1 and suitably terminated inend members (not shown), may provide a relatively rigid suporting meansfor the radiating elem nts without modifying the wave produced.

Similar spacing means are provided intermediate each of the crystalcylinders, the washer 9t and spacers 91 and 92 separating the crystal 15from crystal 14.

It will be noted in Fig. 1 that crystal 15 serves as a transmittingelement and also as a detecting element, both channels 37 and 50 beingconnected thereto. in the broken away section of Fig. 2 is will be seenthat an outer electrode 95, a thin metallic coating on the outer surfaceof cylinder 15, has been milled away as at a grooved section 96 so thata small portion 95a of the electrode 95 is electrically insulated fromthe lower portion.

Similarly the upper portion of the inner electrode 97 may be insulatedfrom the lower portion. However since the inner electrode may beelectrically common to 6 both the driving and detecting portions of thecrystal 15, the inner electrode 97 and conductor 98 are shown aselectrically common to both channels 37 and 50 of Fig. 1. Conductor 99connected to the insulated portion a forms, with conductor 98, thechannel 50 of Fig. 1 while the conductor 100 connected to the lowerportion of the electrode 95, together with conductor 98, forms thechannel 37 of Fig. l. Conductors 99 and 100 are threaded through thewall of the crystal cylinder 15 through bore holes 161 and 102,respectively. The conductors 98, 99 and ltitl together with otherconductors leading to the lower elements 16, 17, 18 and 19 of the systemof Fig. 1 may then be threaded up through the central upper sections ofthe crystals and the intermediate spacing washers.

Since the variations in dimension of crystal 15 as produced byapplication of a signal from conductors 98 and Mill must also produce avariation in the dimensions of the portion contacted by electrode 95a,the voltage generated by such dimensional variations will be directlyproportional thereto and thus available for utilization in the gaincontrol circuit 51, 52 and 53. It will be apparcut that other types ofdetectors may be used but the provision of he grooved cylinder is mostdesirable because of its simplicity and reliability. Strain gauge typedetectors may be utilized. For example, a resistive element may besecured to the periphery of cylinder 15 and connected to the channel 50.Variations in the circumference of crystal 15 would then be sensed bythe strain gauge element. Other such modifications may be found to besuitable.

Referring now to Fig. 4, the power controlling system for the crystal 15has been illustrated along with portions of corresponding circuits forthe adjacent crystals 12 and 13. Where appropriate, the same referencecharacters have been used as in Figs. 1-3. The alternating currentsource 3*? is connected by way of conductor 35a to the signal input gridof a pentode amplifier 110. The pentode amplifier stage is conventionalin construction, its output being coupled by way of condenser 111 to thepower amplifier 112. The output of the power amplifier, having afrequency corresponding to that of the signal from the source 30 isconnected by way of transformer 113 and channel 37 (conductors 98 and100) to the inner and outer electrodes of the crystal 15. It will benoted that the outer surface of the crystal 15 is grooved near the upperend as explained in connection with Fig. 2 so that the upper portion ofthe outer electrode is electrically isolated from the lower portion. Thesignal on channel 37 drives the crystal 15 to produce the cylindricalwave for transmission through the bore hole liquids to adjacentformations. The energy applied to the crystal 15 is measured by thewattmeter [thermocouple element] 45 whose output is applied to therecorder 46.

The amplitude of vibration of the crystal 15, or the magnitude of thewave generated thereby, is detected by a circuit including conductor 99and condenser 1221 connected to the grid of a triode amplifier 122. Theoutput of the triode amplifier 122 is coupled by way of condenser 123 toa voltage amplifier 124. The output of the voltage amplifier is thenapplied by way of condenser 125 to the control grid of a second triodeamplifier 126.

The voltage at the anode of triode 126 is then utilized to derive a DC.biasing voltage to control the gain of the pentode stage 110 and thepower amplifier 112. More particularly, the alternating current signalsat point 130 are applied to the control grid of a triode 131 by way ofcondenser 132. The triode 131 is provided with a grid resistor 131a anda cathode grid-biasing means 133. The grid of tube 131 is at all timesnegatively biased beyond plate current cutoif by means of a variableresistor circuit including resistors 134 and 135. More particularly,current from a source of supply flows from the B+ terminal 136 throughresistors and 134 and then downwardly through the cathode resistor 137to ground, which corre- 7 sponds with the negative side 8-, of thesource of supply. By suitably adjusting resistor 134, the magnitude ofthe current fiowing through resistor 137 may be controlled thereby todetermine the magnitude of the negative bias introduced into the gridcircuit which, of course, includes resistors 137 and 131a. As willhereinafter be shown, a particular setting of the resistor 134 isselected to control at a preselected level the amplitude of vibration ofthe crystal 15.

The anode supply circuit for tube 131 includes a re sistor 140 ofrelatively high value, for example in the order of 500,000 ohms. Thisrelatively high resistance imparts poor voltage-regulation to theanode-supply circuit of the tube 131. In other words, when appliedsignals from tube 126 of predetermined amplitude render the tube 131conductive, the resulting flow of unidirectional current through theresistor 140 produces a larger IR drop so that the point 141 drops to avery low value,

approaching zero as the limit.

' The output from the triode 131 is applied by way of a capacitor 142and conductor 143 to a control network which includes a resistor 144 inone branch of the circuit, a diode rectifier 145 and a resistor 146 inanother branch of the circuit, both of said branches leading to a groundterminal 147. It will be at once apparent that the charging circuit forthe capacitor 142 may be traced from the positive source of B+ supply,the point 136, through resistor 140, capacitor 142, conductor 143 andresistor 144 to ground.

Though this charging circuit has been traced as between 13+ and B- it isto be understood that the capacitor 142 is under the control of tube131. When tube 131 is rendered conductive by a signal above apredetermined magnitude at the anode point 130 of tube 126, the voltageat point 141 drops to a small value. Hence the capacitor 142 dischargesthrough resistor 144 and through the secnd branch of the circuitconnected in parallel therewit which includes the diode 145 and theresistor 146. On the other hand, when the tube 131 is non-conductive,the voltage at point 141 is high and the capacitor 142 charges.

Due to the unidirectional or non-linear character of the rectifier 145the current flowing through the second branch of the circuit, therectifier 145 and the resistor 146, during the charging of the capacitor142 is negligible. However the diode 145 does provide a discharge pathparallel to resistor 144 for the condenser 142 which may be traced frompoint 141 through tube 131 and the bias circuit 133 to ground, andthence through resistor 146 (of approximately 2 megohms), the diode 145and the other side of capacitor 142. Thus upon discharge of thecondenser 142 a portion of the current flows through resistor 146 in adirection such that the side adjacent the point 150 is negative withrespect to ground.

The negative voltage appearing at point or as measured by meter 146a isthen applied by way of a smoothing circuit 151 and a resistor 152 to thesuppressor grid of the pentode amplifier stage 110. The voltage may alsobe applied by way of a circuit including resistor 153 to the poweramplifier 112, in a gain-controlling relation, so that the powerdelivered to the transformer 113 from the source is under the solecontrol of the voltage appearing between the conductor 99 connected tocrystal 15 and a ground terminal.

In Fig. 4 it will also be seen that the crystal 18 is provided with asimilar circuit, shown only in block diagram form with a fragmentaryshowing of the gain-power control network identical in construction andoperation with the more detailed circuit shown for the crystal 15. Thecircuit for crystal 18 includes a variable resistor 134a whose functionis identical with respect to the crystal 18 as resistor 134 with respectto crystal 15. The elements 140a, 142a, 135a similarly correspond to theelements of the circuit of crystal 15 having the same referencecharacters minus the sufiix. A similar circuit, not shown, is providedfor control of crystal 12. By adjustment of the setting of the variableresistor 134, 134a and the corresponding resistor for the circuit of thecrystal 12 the amplitude of vibration of all of the crystals in theradiating unit, Fig. 1, are initially adjusted to the same level whenthe radiating unit is in a homogeneous medium such as in air or in aliquid filled bore hole at a point where the formation is homogeneous.Thereafter when the radiating unit is lowered past formations ofdiffering character, the variations in the power requirements formaintaining constant the amplitude of vibration and thus maintaining thewave cylindrical in the region of crystals 14, 15, 16, will be sensed bythe wattmeter [thermocouple element] 45 to produce a useful log. It isto be noted that the initial adjustment of the controlling resistors orthe power delivered to the different radiating sections will not beexactly the same since the power requirements for the end or the guardsections comprised of crystals 11, 12, 13, 17, 18 and 19 will bedifierent [diiferet] than for the central or probe section 14, 15 and 16(Fig. 1). This is for the reason that there is end radiation associatedwith the end sections not present in the center section.

The foregoing gain control circuit has been presented by way ofillustration only and not by way of limitation. Other forms of gaincontrolling circuits may be applied to control the magnitude of thevibration of. the radiating elements as disclosed by applicant.

While the foregoing description relates specifically to the utilizationof low frequency sinusoidal cylindrical waves, it will be apparent thatother types of excitation may be used to advantage by providing theacoustic end guards driven in a predetermined relation with respect toan intermediate measuring or sensing system. Acoustic pulses may befound useful in contrast with the sinusoidal phenomena for whichEquation 1 is valid. Still other modes of operation may also be found tobe suitable in the production of acoustic logs of the acousticproperties of earth formations adjacent liquid filled bore holes by useof the end shielded system herein disclosed.

In Fig. 5 there is disclosed a system which may be used in place of thecrystal system of Fig. 1. For the purpose of simplicity, only onesection has been shown and where appropriate, elements performing thesame function as the elements of Fig. 1 have been given the samereference character with the suffix a.

Fig. 5 includes an elongated cylindrical structure comprised of aplurality of flat rings 161 stacked end-toend and suitably boundtogether as between a pair of end rings 162 and 163 of insulatingmaterial, such as Micarta or Bakelite. The rings 161 are ofmagnetostrictive material producing radial displacements upon excitationby magnetic flux for production of vibrational motion in surroundingmedium. In this case, as in Fig. l, a rubber sheath 164 is mouldedaround theexterior of the tubular member 160 for insulating it fromfluids and for transmission of motion to the surrounding medium.

An alternating source 30a is connected by way of circuit 35a through anamplifier 36a and an input circuit 37a to a toroidal winding 165 on theelongated cylinder 160. While but a few turns have been shown ascomprising the winding 165, it will be apparent that the entireperiphery of the cylinder 160 may be covered with windings for producingan optimum flux density in the magnetostrictive elements for a givenoutput from amplifier 36a. If desired, a unidirectional field may beimposed upon the magnetostrictive elements by application of a directcur rent component to the excitation of the winding 165. Such excitationhas not been shown in Fig. 5, but the manner of so providing is wellunderstood by those skilled in the art. However, the [remanant] remanentmagnetism of the magnetostrictive element is sufficient that alternatingcurrent alone applied to the windings 165 produces desired radialdisplacements for production of acoustic Waves. As in Fig. 1, awattmeter [thermocouple element] 45a is connected to a recorder 46a toproduce a record of the power parameter of the excitation of themagnetostrictive elements 161.

A detector Winding 170 is coupled in inductive relation to themagnetostrictive elements for detecting the flux density in themagnetostrictive elements as a measure of displacement. Sincedisplacements are proportional to the derivative of flux density, thevoltage induced in coil 170 must be integrated to produce an outputvoltage which is a measure of displacement. The integrating amplifier171, of the types known to the art, produces an output function forapplication to the automatic volume control circuit 53a. The lattercircuit, as explained in connection with Fig. 1, may control the outputof the power amplifier 36a for maintaining the acoustic level in mediaadjacent the cylinder 16d at a constant level.

The auxiliary circuits and additional driving cylinders shown in Fig. 1may be substantially duplicated with magnetostrictive elements of thetype illustrated in Fig. 5. The system of Fig. 5 may be utilized inplace of the elements of Fig. l with an added advantage that themagnetostrictive elements are not temperature sensitive and may possiblybe operated in certain well logging operations where temperatures tooextreme for crystal operation are encountered.

It will now be apparent that a single toroid of substantial length, or12 hole diameters, may be excited from a single source as illustrated inFig. 5, and measurement made of the displacement of a short centralsection, as by inductively coupling a pick-up coil, such as the coil170, to a limited number of centrally located magnetostrictivelaminations for measurements of displacements, or amplitude ofvibrational energy transmitted to adjacent formations as by meter 172which indicates the magnitude of the AVG voltage or by recording theenergy function on the recorder 46a. The coil 178 preferably is coupledto the portion of the driven member at its center as to be substantiallyunafi'ectcd by end radiation effects. it will further be apparent thatdata concerning the prop erties of the formations new and distinct fromthat produced by prior art systems may be obtained by measurement of themagnitude of the voltage in coil 170, such coil occupying but a portionof the central laminations of the transducer 160 if a constant amplitudevoltage is applied from source 3iia to winding 165 as contrasted to themode of operation in which the energy is maintained constant.

While preferred embodiments of the invention have been described, itwill be understood that other modifications may now suggest themselvesto those skilled in the art, and it is intended to cover suchmodifications as fall within the scope of the invention set forth in theappended claims.

What is claimed is:

1. In acoustic logging where an exploring element is used to log aliquid filled bore hole, the method which comprises driving said elementto generate an axially symmetric wave, with said element in ahomogeneous medium, independently adjusting the amplitude of said waveat different points along said element to a predetermined level, movingsaid element through said bore hole and past the formations adjacentthereto to modify said wave over the length of said element independence upon acoustic properties of said formations, and generating asignal representative of variations in said wave in a preselectedintermediate fraction of the length of said element for determination ofvariations in said acoustic properties.

2. In acoustic logging where an exploring element is used to log aliquid filled bore hole, the method which comprises driving said elementto generate an axially symmetric cylindrical wave acoustically to couplethe formations adjacent said borehole to said element, independentlyadjusting the magnitude of said wave at differe'nt points along saidelement to a predetermined level moving said element through said borehole and past the formations adjacent thereto to vary the energyrequired to maintain constant the amplitude of said wave over the lengthof said element in dependence upon acoustic properties of saidformations, and generating a signal representative of variations in saidenergy supplied to a preselected intermediate fraction of the length ofsaid element for determination of variations in said acousticproperties.

3. In acoustic logging where an exploring element is used to log aliquid filled bore hole, the method which comprises driving a firstportion of said element for generation of an axially symmetric waveacoustically to couple formations adjacent said first portion to saidelement, separately driving adjacent portions of said element above andbelow said first portion for generation of axially symmetric waves inphase with the wave from said first portion to couple adjacentformations tc said adjacent portions of said element, independentlyadjusting the amplitude of the Waves from all of said portions tosubstantially the same level to assure a Wave closely purely cylindricalin form adjacent said first portion, moving said element through saidbore hole and past the formations adjacent thereto to modify the wavesfrom said portions of said element in dependence upon acousticproperties of adjacent formations, and generating a signalrepresentative of variations in the wave from said first portion fordetermination of variations in said acoustic properties of formationsadjacent said first portion.

4. In acoustic logging Where an exploring element is used to log aliquid filled bore hole, the method which comprises driving a firstportion of said element for generation of an axially symmetric waveacoustically to couple formations adjacent-said portion to said element,separately driving adjacent portions of said element above and belowsaid first portion for generation of axially symmetric Wavescorresponding in phase with the wave from said first portion to coupleadjacent formations to further portions of said element, independentlyadjusting the magnitude of the waves from all of said portions tosubstantially the same magnitude to assure a cylindrical wave informations adjacent said first portion, moving said element through saidbore hole past the formations adjacent thereto to vary the energyrequired to maintain constant the amplitude of said waves from each ofsaid portions in dependence upon acoustic properties of formationsadjacent the respective portions, and generating a signal representativeof variations in said energy supplied to said first portion fordetermination of variations in said acoustic properties.

5. A system for measuring changes in the acoustical character offormations penetrated by a liquid filled bore hole which comprises anexploring element which includes a plurality of cylindrical radiatingmembers supported in end-to-end relation, means for separately drivingsaid cylindrical members in phase for generation of axially symmetricacoustic Waves, control means for maintaining the amplitude of saidwaves from each of said radiating members at a preselected value toproduce a wave truly cylindrical in character along at least anintermediate member of said exploring element, means for moving saidexploring element through said bore hole to modify the portions of saidWave from said individual radiating members in dependence upon theacoustic properties of formations immediately adjacent each portion, andmeans for measuring variations in the wave from said intermediate memberfor determination of variations in the acoustic properties of formationsadjacent thereto.

6. A system for measuring changes in the acoustical character offormations penetrated by a liquid filled bore hole which comprises afirst radiating member, a source of low frequency alternating current, afirst power controlling network connected between said radiating memberand said source for driving said member to produce 11 a low frequencyaxially symmetric sinusoidal wave, a second radiating member adjacentone end of said first member, a power controlling networkinterconnecting said source and said second radiating member, a thirdradiating member adjacent the other end of said first member, a thirdpower controlling network interconnecting said source and said thirdmember, means responsive to the vibration of each of said members forproducing alternating current voltages, gain control means respectivelyresponsive to each of said voltages and coupled to the power controllingnetworks in a gain controlling relation for individually controlling theamplitude of the vibration of each of said members, means in each ofsaid gain control means for adjusting the amplitudes of vibrations ofassociated members to a preselected value to produce a wave cylindricalin character along the entire length of said first member, means formoving said elements in end-to-end relation through said bore hole tomodify the energy required from each of said power controlling networksto maintain constant the amplitudes of said waves, and means formeasuring variations in the energy applied to said first member fordetermination of variations in the acoustic properties of formationsadjacent said first member.

7. An acoustic well logging system for measuring properties offormations penetrated by a liquid filled well bore which comprises aplurality of piezoelectric elements supported in an end-to-end array anddimensionally variable in a direction normal to the longitudinal axis ofsaid array, driving means connected to said elements for separatelydriving said elements for producing such dimensional variations, meansfor independently adjusting the dimensional variations of said elementsto substantially the same magnitude, means for moving said elements insaid end-to-end relation through said bore hole for transmission throughsaid liquid to said formations of acoustic waves due to said dimensionalvariations longitudinally uniform adjacent at least the centrallylocated elements, and detecting means coupled to one of said centrallylocated elements whereby measurement of said waves may be madeindependently of radiation along the axis of said well bore from theelements at the ends of said array.

8. An acoustic Well logging system for measuring properties offormations penetrated by a liquid filled well bore which comprises aplurality of cylindrical piezoelectric elements supported in anend-to-end array and di mensionally variable in a direction normal tothe longitudinal axis of said array, driving means connected to saidelements for separately driving said elements for producing suchdimensional variations, means for independently adjusting thedimensional variations of said elements to substantially the samemagnitude, means for moving said elements in said end-to-end relationthrough said bore hole for transmission through said liquid to saidformations of acoustic waves due to said dimensional variationscylindrical adjacent at least the centrally located elements, anddetecting means coupled to one of said centrally located elementswhereby meas urement of said Waves may be made independently ofradiation along the axis of said well bore from the elements at the endsof said array.

9. An acoustic well logging system for measuring prop erties offormations penetrated by a liquid filled well bore which comprises aplurality of transducers stacked in an end-to-end array and variabledimensionally in a direction normal to the axis of said array, drivingmeans for separately exciting each of said transducers to produce suchdimensional variations, means for independently adjusting thedimensional variations of said elements to substantially the samemagnitude, supporting means for moving said transducers through saidbore hole for trans- 7 mission through said liquid to said formations ofacoustic waves due to said dimensional variations uniform in nature atleast adjacent the centrally located transducers,

and detecting means coupled to one of said centrally located transducerswhereby said waves may be measured independently of radiation from thetransducers positioned at the ends of said array.

10. A system for measuring changes in the acoustical character offormations penetrated by a liquid filled bore hole which comprises anexploring element including 'a plurality of radiating members supportedin end-to-end relation,means for driving said plurality of radiatingmembers in phase for generation of axially symmetric acoustic waves,automatic gain control means connected between selected ones of saidradiating members and said driving means and responsive to themagnitudes of the waves from each of said members respectively tocontrolthe driving means for each of said members and for maintainingsubstantially at said preselected value the ampli tude of the wave overthe entire length of said element, means for adjusting the amplitude ofsaid waves generated byeach of said members to said preselected value topro duce a wave cylindrical in character along an intermediate fractionof the length of said exploring element, means for moving said exploringelement through said bore hole to modify the portions of the wave fromsaid individual radiating members in dependence upon the acousticproperties of formations immediately adjacent each portion, saidautomatic gain control means being responsive to modification of saidwaves from said intermediate fraction to vary said driving means toreturn the amplitude of said waves to said preselected value and meansfor measuring variations in the driving means for the members in saidintermediate fraction for determination of variations of the acousticproperties of for mations adjacent said fraction.

11. A system for measuring changes in the acoustical character offormations penetrated by a liquid filled bore hole which comprises anexploring element which includes an intermediate radiating member andradiating members supported at each end of said radiating member in anend-to-end relation, low frequency means for driving said radiatingmembers in phase for generation of axially symmetric acoustic waves,automatic gain control means responsive to the amplitudes of vibrationof said radiating members respectively connected in gain controllingrela- 'tion to the low frequency driving means for said members tomaintain constant the amplitudes of the vibration of the waves from saidmembers, means for adjusting the amplitude of the waves from each ofsaid radiating members to said constant amplitude to produce a wavecylindrical in character at least along the fraction of the length ofsaid exploring element corresponding to the location of saidintermediate radiating member, means for moving said exploring elementthrough said bore hole to modify the portions of said wave from saidradiating members in dependence upon the acoustic properties offormations immediately adjacent each member, said automatic gain controlmeans being responsive to modification of said waves to vary the energyoutput of said low frequency driving means to maintain constant theamplitude of the waves from said intermediate member, and means formeasuring variations in the energy required to maintain constant theamplitude of the wave from said intermediate member for determination ofvariations in the acoustic properties of formations adjacent saidintermediate member.

12. An acoustic well logging system for measuring properties offormations penetrated by a liquid filled well bore comprising a housingadapted to be lowered into the well bore, a plurality of transmittingelements mounted within said housing and comprised 0, bariumlitanatecylinders disposed coaxially with said housing, electrodes disposedwithin said housing, one electrode being disposed about the insideperiphery of each cylinder and a second electrode being disposed aboutthe outside periphery of each cylinder, means for causing simultaneouscontinuous expansion and contraction of said barium-titanate cylindersfor sending continuous acoustic waves through said formations, detectingmeans located coaxially within said housing and between transmittingelements, and means for obtaining an electrical signal from saiddetecting means indicative of the type of formations traversed by saidhousing.

13. An acoustic well logging system as in claim 12 in which saiddetecting means includes a barium-titanate member having inside andoutside surfaces, said member being positioned within said housing andcoaxially there with, an electrode disposed on the inside surface ofsaid member and an electrode disposed on the outside surface of saidmember.

14. An acoustic well logging system for measuring properties offormations penetrated by a liquid filled well bore which comprises aplurality of crystal transducers stacked in an en -to-end array andvariable dimensionally in a direction normal to the axis of said array,driving means for separately exciting each of said transducers toproduce such dimensional variations, means for adjusting the dimensionalvariations of said transducers to substantially the same magnitude,supporting means for moving said transducers through said bore hole fortransmission through said liquid to said formations of acoustic wavesdue to said dimensional variations uniform in nature at least adjacentthe centrally located transducers, and detecting means positionedcentrally of said transducers for generation of an electric signalindependent of radiation from the transducers positioned at the ends ofsaid array.

15. An acoustic well logging system for measuring properties offormations penetrated by a liquid filled well bore comprising a housingadapted to be lowered into the well bore, a plurality of transmittingelements mounted within said housing and comprised of piezoelectriccylinders disposed coaxially with said housing, electrodes disposedwithin said housing, one electrode being disposed about the insideperiphery of each cylinder and a second electrode being disposed aboutthe outside periphery of each cylinder, means for causing simultaneouscontinuous expansion and contraction of said piezoelectric cylinders forsending continuous acoustic waves through said formations, detectingmeans located coaxially within said housing and between transmittingelements, and means for obtaining an electrical signal from saiddetecting means indicative of the type of formations traversed by saidhousing.

16. An acoustic well logging system for measuring properties offormations adjacent a bore hole comprising a housing adapted to belowered into the bore hole, a piezoelectric sound transmitting meansmounted within said housing and including at least one piezoelectrictransmitter, a piezoelectric detector mounted within said housing, thearrangement being such that the longitudinal extremities of the soundtransmitting means extend above and below the longitudinal extremitiesof the piezoelectric detector, means for causing the continuousexpansion and contraction of said piezoelectric sound transmittingmeans, and means for obtaining an electric signal from saidpiezoelectric detector indicative of the type of formations traversed bysaid housing.

References Cited in the file of this patent or the origmal patent UNITEDSTATES PATENTS 2,190,686 Slichter Feb. 20, 1940 2,405,187 Beniotf Aug.6, 1946 2,434,648 Goodale Ian. 20, 1948 2,530,971 Kean Nov. 21, 19502,586,745 Tullos Feb. 19, 1952 2,633,484 Zimmerman Mar. 31, 19532,649,163 Atkins Aug. 18, 1953 2,894,597 Kean et al. July 14, 1959

